[Ur] solution to the DB chicken and egg problem?

[Adam Chlipala]

I'm not familiar with a standard database feature to allow that mode, 
but it sounds like you are referencing an extra annotation on 
constraints.  Somehow this situation has only come up once in the 10+ 
years of Ur/Web's existence, and it's never been an issue for my own 
Ur/Web apps.

On 04/02/2017 10:10 AM, Marko Schütz Schmuck wrote:
> On Sun, 02 Apr 2017 09:02:15 -0400,
> Adam Chlipala wrote:
>> Actually, Ur/Web won't even accept those table definitions: no mutually recursive
>> definitions yet, w.r.t. constraints.  And I haven't thought before about allowing
>> temporary breaking of constraints.
> "Constraints hold after every statement" -> "Constraints hold after
> every transaction"?
>
>> On 04/02/2017 08:35 AM, Marko Schütz Schmuck wrote:
>>> If I have tables
>>>
>>> table chicken : { Id : int, Egg : int }
>>>         PRIMARY KEY (Id),
>>>         CONSTRAINT egg FOREIGN KEY Egg REFERENCES egg(Id)
>>>
>>> table egg : { Id : int, Chicken : int }
>>>         PRIMARY KEY (Id),
>>>         CONSTRAINT chicken FOREIGN KEY Chicken REFERENCES chicken(Id)
>>>
>>> Outside of Ur/Web this would be solved by setting the constraints
>>> DEFERRABLE INITIALLY DEFERRED and do the INSERTs pairwise inside a
>>> single transaction.
>>>
>>> I doubt that there currently is a way to do this in Ur/Web since there
>>> is neither a way to specify DEFERRABLE INITIALLY DEFERRED nor a way to
>>> explicitly form transactions.
>>>
>>> Would it be enough to introduce an operator
>>>
>>> dml_sequence : dml -> dml -> dml
>>>
>>> allowing two dml statements to be composed and run in a single
>>> transaction together with exposing DEFERRABLE INITIALLY DEFERRED at
>>> the Ur/Web language level?
>>>
>>> Best regards,
>>>
>>> Marko
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